Remove true/false return from action.execute()

In watcher/applier/workflow_engine/default.py, we are checking the
return value of action.execute(). As the "TODO" above indicates it
(line 118), we should get rid of this and only flag an action as
failed if an exception was raised during its execute(). We will
need to update the related unit tests.

Change-Id: Ia8ff7abd9994c3504e733ccd1d629cafe9d4b839
Closes-Bug: #1548383

watcher/common/exception.py

@@ -234,6 +234,9 @@ class PatchError(Invalid):
 
 # decision engine
 
+class WorkflowExecutionException(WatcherException):
+    msg_fmt = _('Workflow execution error: %(error)s')
+
 
 class IllegalArgumentException(WatcherException):
     msg_fmt = _('Illegal argument')